Intermediates useful in the synthesis of alpha-lipoic acid



2,809,978 Patented O t; 1 il 7:

INTERMEDIATES USEFUL IN THE SYNTHESIS OF a-LIPOIC ACID Frederick W. Holly, Cranford, Arthur F. Wagner, Princeton, and Edward Walton and Carl H. Hotfmau, Scotch Plains, N. J., assignors to Merck & Co., Inc., Rahway, N. J., a corporation of New Jersey No Drawing. Application December 4,

Serial No. 396,334

19 Claims. (Cl. 260-399) This invention is concerned with the production of novel 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acids having the formula SR R and salts and esters thereof, wherein R and R are the same or different hydrocarbon substituents such as alkyl, alkenyl, alkynyl, aryl and aralkyl groups. This invention is also concerned with novel processes for producing these compounds in which org-(hydrocarbon substituted mercapto) butyryl halides are used as the starting materials. In addition, this invention is concerned with novel intermediates useful in producing the 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acids.

The novel 6,8-(hydrocarbon substituted mercapto)-5- oxocaprylic acids are valuable in the production of other novel chemical compounds and are particularly important as precursors in the synthesis of u-lipoic acid or 5-[3- (1,2-dithiacyclopentyl) Jpentanoic acid as will be shown hereinafter. In this pentyl)]. pentanoic acid is a valuable growth stimulating crystalline substance which was first obtained from liver and later found to have the structure stituted mercapto)-2-carboxy-3-oxocaproic acid. This initial reaction is illustrated as follows:

ll CHzCHzCHC GHQ R orncrnonoox 00012 as R YCH SR R 000R:

I cooR 1 11 III wherein X represents a halogen, Y represents an alkali metal, alkaline earth metal or alkoxymagnesium and R, R, R and R represent the same or different hydrocarbon substituents such as alkyl, alkenyl, alkynyl, aryl and aralkyl groups.

The diesters of 4,6-(hydrocarbon substituted mercapto)-2-carboxy-3-oxocaproic acids are then converted to the. corresponding 4,6-(hydrocarbon substituted merregard, 5 [3-(1,2 dithiacyclocapto)-3-oxocaproic acids and esters thereof according to the reactions:

0 I 5 a orncrnondomooolv a 2' SR SR n A 7 IV iornomoaconooon l R sa 00R: 10

III II cmornouoomooon SR .SR

rv-a

1 wherein R, R, R and ly assigned. a

The 4,6-(hydrocarbon substituted mercapto)-3-oxocaproic acids are subsequently reacted in the form of an ester or salt with a suitable derivative of acrylic acid such as anamide, nitrile, ester or salt to form the corresponding derivative or 6,8-(hydrocarbon substituted mercapto)-4-carboxy-5-oxocaprylic acid, which reaction can be illustrated as follows:

R have the significance previous ornomoudomooom oHFoHR SR SR V VI CHQCHtCHCCHCHzCHzR SR sa 003 wherein R and R have the meaning previously assigned, R represents an alkyl, aryl or aralkyl group or a metal and R represents the cyano, carbarnyl and COOR groups wherein R has the same significance assigned herein.

The fourth and last reaction comprises a simultaneous hydrolysis and decarboxylation of the derivatives of the 6,8 (hydracarbon substituted mercapto) 4 carboxy 5- oxocaprylic acids to produce the desired 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acids as illustrated by the following reaction:

' omcmoaconcmcmom CHzCHgCHO CH9sOOOH R R 00011 SR SR VII VIII wherein R, R, R and R have the meaning given above.

The initial reaction of this process in which an u/y- (hydrocarbon substituted mercapto)-butyryl halide (I) is reacted with a salt of a malonic acid diester (II) to prepare a diester of 4,6-(hydrocarbon substituted mercapto)- 2carboxy-3-oxocaproic acid (III) may be conveniently achieved by contacting the reactants in the presence of an inert solvent. For this purpose, inert solvents such as the lower alkyl ethers like ethyl ether and isopropyl ether, chloroform, saturated hydrocarbons such as pentane and 0 hexane,.benzene and toluene are particularly suitable,

preferably when substantially free of water. The mixture of reactants in a suitable solvent is ordinarily heated to an elevated temperature to effect the reaction and, in this regard, the reflux temperature may be used with 55 good results. Under these conditions the reaction is completed in less than one hour. Following completion of this reaction. the desired diester of 4,6-(hydrocarbon substituted mercapto)-2-carboxy-3-oxocaproic acid may be isolated from the reaction mixture by conventional methods such as distillative removal of the solvent. It is preferred however, not to isolate this product from the reaction mixture but, instead, to immediately subject the product as present in the reaction mixture to conditions which effect decarboxylation of the Z-carboxy group according to methods hereinafter disclosed.

The described reaction may be conveniently effected using any suitable a, -(hydrocarbon substituted mercapto) butyryl halide as the starting material. Thus, the 11, hydrocarbon substituents may be the same or different alkyl, alkenyl, alkynyl, aryl and aralkyl radicals. In this regard, it is" generally preferred to employ those compounds as starting materials which have identical hydrocarbon substituents. Such compounds are referred to herein as bis-substituted. In addition, the a,'y'-bis(alkylmercapto and alkenylmercapto) butyryl halides are particularly useful in the synthesis of ec-liPOiC acid. Specific examples of hydrocarbon substituents which may be present on'these starting materials are methyl, ethyl, propyl, butyl, t-butyl, allyl, butenyl, pentenyl, heptenyl, ethynyl, propynyl-2, phenyl, benzyl and similar groups. In addition ,tthis reaction is preferably achieved with halides of the type describedin which the halogen substituent is either-chlorine or bromine.

- Salts of malonic acid diesters which may be used in this invention that might be mentioned are the alkoxymagnesium derivatives of malonic acid diesters or the alkali metal and alkaline earth metal enolates. The ester groupsofsuch salts may be either the same or different alkyl, aryl and aralkyl groups. Some specific examples of such salts which may be used as reactants in this process are dimethyl-methoxymagnesium malonate, diethyl-ethoxyma'gnesium malonate, diphenyl and dibenzylethoxymagnesium malonate, ethyl t-butyl ethoxymagnesium malonate, and sodium, potassium, calcium and magnesium salts of dimethyl, diethyl, diphenyl, dibenzyl and similar mixed esters of malonic acid.

The reaction described may be used to prepare any desired diester of 4,6-(hydrocarbon substituted mercapto)- 2-carboxy-3-0Xocaproic acid. In a specific illustration of this reaction a,' -bis(methylme'rcapto) butyryl chloride is reacted with ethyl t-butyl ethoxymagnesium malonate to obtain tertiary butyl 4,6-bis(methylmercapto)-2-carbetho'Xy-3 oXcaproate. Examples of similar compounds which'are prepared in this manner from the appropriate reactants are methyl-4,6-bis(ethylmercapto)-2-carbomethqxy 3 oxocaproate, ethyl-4,6-bis(ethylmercapto)-2-ca-r be'thoxy 3 oxocaproate, ethyl-4,6-bis(allylmercapto)-2- carbeth'oxy-3 oxocaproate, methyl- 4,6 bis(2'-methyl-1- ethyl 2 butene mercapto)-2-carbethoxy 3 oxocaproate propyl 4,6 bis(3-methyl-2-ethyl-2-pentene mercapto)-2- carbomethoxy-3-oxocaproate, phenyl 4,6-bis (phenylmercapto).-2-carbophen'oxy 3-oxocaproate, ethyl-4,6 bis(benzylmercapto) 2 carbethoxy-3-oxocaproate, benzyl-4,6- bis (propargylmercapto -2-carbobenzyloXy-3-oxocaproate, methyl-4-ethylmercapto-6-propylmercapto-2 carbometh- 'oxy-3 -oxocaproate, and the like.

According to the second step of this process the die st'ers of the 4,6-(hydrocarbon substituted mercapto)-2-carboxy- 3-oxo'caproic acids (III) produced above are converted to the corresponding mono-esters of 4,6-(hydrocarbon substituted mercapto) -3-oxo caproic acid (IV). This conversion inwhich the 2-carboxy groupis eliminated from the caproic acid moiety is attained by either an acid catalyzed reaction or by thermal decarboxylation to. the desired product.

Acid catalyzed decarboxylation is" preferably achieved under solvent conditions in the presence of a suitable acid. Thus, this reaction is' conveniently effected'b'y contacting the diester of the 4,6-(hydrocarbon substituted mercapto) 2-carboxy-3-oxocap'roic acid in inert solvents such as ethers', benzene, hexane, chloroform and the like with' catalyticamounts of acids such as alkyl and aryl sulfonic' acids and particularly para-toluene sulfonic acid. In this regard, the use of substantially anhydrous conditions is sometimes preferable for optimum results. Elevated temperatures above 50 C. and up to the reflux temperature are used for the decarboxylation. Under such conditions the reaction is completed in an hour or two and the product formed may be recovered from the reaction mixture by the usual methods. Thus, the solvent may be evaporated and the oily product purified by distillation under reduced pressure.

More specifically, this transformation is illustrated by the conversion of t-butyl-4,6 -(methylmercapto)-2-carboethoxy-3-oxocaproate in benzene at reflux temperature to ethyl-4,6-bis(methylmercapto)-3-oxocaproate. Representative of other similar compounds which are produced according to this decarboxylation reaction from the appropriate reactants are r'nethyl-4,6-bis(ethylmercapto)-3-oxocaproate, ethyl 4,6 bis(ethylm'ercapto) 3-oxocaproate, ethyl-4,6-bis(allylmercapto)-3-oxocaproate, methyl 4,6- bis(2-methyl-1-ethyl 2-butene mercapto)-3-oxocaproate, propyl-4,6-bis(3-methyl-2-ethyl-Z-pentene mercapto) 3- oxocaproate, phenyl-4;6 bis(phenylmercapto) -3-oxocaproate, ethyl-4,6-bis(benzylmercapto)-3-0Xocaproate, benzyl- 4,6-bis(propargylmercapto)-3-oxocaproate, methyl-4-ethylrnercapto';6-propylmercapto-3-oxocaproate and the like.

As has been indicated above, this decarboxylation is also effected by thermal cracking. According to this procedure diesters of the 4,6-(hydrocarbon substituted mercapto)-2-carboxy-3-oxocaproic acids are heated at a suitable elevated temperature, preferably in the neighborhood of about ISO-300 C.,- to accomplish removal of the 2-carboxy group and form the corresponding mono-esters. After the conversion has been accomplished the product may be recovered byconventional methods and purified. By proper selection of the reactants those mono-esters previously named herein and other similar compounds can be produced by this procedure.

The esters of 4,6-(hydrocarbon substituted mercapto)- 3-oxocaproi-c acids are readily converted to the corresponding alkali metal and alkaline earth metal salts such as sodium, potassium, calcium and magnesium salts by hydrolysis of the ester with an alkali or alkaline earth metal base. The salts in turn are converted to the free acids'by hydrolysis with mineral acids" such as hydrochloric acid andsulfuric acid.

The third reaction of this process is directed to the production of various derivatives of 6,8-(hydrocarbon substituted mercapto)-4-carboxy-5-oxocaprylic acids such as nitriles, amides, diesters and dimetal salts thereof (VII). Such compounds may be conveniently prepared by reacting an ester or salt of a 4,6-(hydrocarbon substituted mercapto)'-3-oxocaproic acid (V) prepared as above with a derivative of acrylic acid (VI) under suitable reaction conditions. This reaction may be readily effected by contacting the reactants in the presence ofa suitable so'lvent, preferably 'an alcohol such as the lower alcohols like-methanol" and ethanol; Ordinarily a small amount of a quaternary ammonium hydroxide such as trirnethyl benzyl ammonium"hydroxide (Triton B) or an alkali metal alkoxide is added to the mixture to promote the reaction.- To prevent undue side effects the temperature is ordinarily controlled below 60 C. After the reactants have been combined, preferably using approximately equimolar amounts of each reactant, the mixture is continually s'tirredfor 12 to 60 hours during which time an elevated temperature below 60 C. is maintained. Upon completion of the reaction the desired product may be recovered from the reaction mixture by extraction with an immiscible solvent such as chloroform, evaporation of the solvent andi'distillation of the product under reducedpressure.

This reaction is conveniently effected with any suitable derivative of acrylic acid such as acrylonitrile, acrylamide and esters-and salts of acrylic acid such as the alkyl, aryl and aralkyl esters and-alka1i metal and alkaline earth metal salts thereof. More specifically,*the methyl, ethyl,

propyl, butyl, phenyl and benzyl esters and sodium, potassium, magnesium andcalcium salts of acrylic acid are examples of compounds which may be used in this reaction.

Specifically illustrative of this method is the reaction of ethyl-4,6-bis(methylmercapto)-3-oxocaproate with methyl acrylate to produce methyl-6,8-bis(methylmercapto)-4-carbethoxy-5-oxocaprylate. Other examples of closely related compounds that are incorporated within the scope of this invention which are produced according to this method from the appropriate reactants are ethyl- 6,8 -bis(ethylmercapto) 4 carbomethoxy -oxocaprylate, ethyl 6,8 bis(ethylmercapto) 4 carbethoxy 5 oxocaprylate, methyl 6,8 bis(methylmercapto) 4 carbomethoxy 5 oxocaprylate, phenyl- 6,8 bis(phenylmercapto) 4 carbethoxy 5 oxocaprylate, benzyl 6,8 bis (propylmercapto) 4 carbophenoxy 5 oxocaprylate, methyl 6,8 bis(allylmercapto) 4 carbethoxy 5 oxocaprylate, ethyl 6,8- bis(2 methyl 1 ethyl 2 butene mercapto) 4- carbomethoxy 5 oxocaprylate, propyl 6,8 bis(3- methyl 2 ethyl 2 pentenemercapto) 4 carbopropoxy 5 oxocaprylate, ethyl 6,8 bis(propynylmercapto) 4 carbethoxy 5 -oxocaprylate, methyl- 6 ethylmercapto 8 propylmercapto 4 carbomethoxy- 5-oxocaprylate and alkali metal and alkaline earth metal salts such as the sodium and potassium, calcium and magnesium salts and the nitrile and amide of these and similar compounds. The free acids of such compounds are readily obtained by hydrolysis of the nitrile, amide, esters and salts according to conventional methods.

The last reaction in this process comprises the combined hydrolysis and decarboxylation of a derivative of 6,8-(hydrocarbon substituted mercapto)-4-carboxy-5-oxocaprylic acid (VII), such as described above, to the corresponding 6,8-(hydrocarbon substituted mercapto)-S- f oxocaprylic acid (VIII). This reaction may be conveniently carried out by treating the reactant with a suitable mineral acid, preferably inthe presence of a carboxylic acid. The reaction, however, is preferably run in a mixture of glacial acetic acid and hydrochloric acid since the product can be readily recovered by distillation of such acids. The reaction proceeds at room temperature although elevated temperatures are also used with satisfactory results. Under such conditions the reaction is completed in from about 6 to about 60 hours. The reaction product is readily recovered from the mixture according to conventional methods such as evaporation and extraction with a suitable solvent such as chloroform.

The described reaction is specifically illustrated by the conversion of methyl-6,8-bis(methylmercapto)-4-carbethoxy-S-oxocaprylate in the presence of a mixture of hydrochloric acid and glacial acetic acid to 6,8-bis(methylmercapto)-S-oxocaprylic acid. In a similar manner other compounds such as 6,8-bis(ethylmercapto)-5-oxocaprylic acid, 6,8-bis(propylmercapto)-5-oxocaprylic acid, 6,8- bis(butylmercapto)-5-oxocaprylic acid, 6,8-bis(phenyl mercapto) -5-oxocaprylic acid, 6,8-bis (benzylmercapto S-oxocaprylic acid, 6,8-bis(allylmercapto)-5-oxocaprylic acid, 6,8-bis (2-methyl-1-ethyl2-butene mercapto)-S-oxocaprylic acid, 6,8-bis (3-methyl-2-ethyl-2-pentene mercapto)-S-oxocaprylic acid, 6,8-bis (propargylmercapto)-5- oxocaprylic acid, 6-ethylmercapto-8-propylmercapto-5- oxocaprylic' acid and the like are prepared by the proper selection of reactants.

Esters of the 6,8-(hydrocarbon substituted mercapto)- 5 -oxocaprylic acids are conveniently prepared by reacting such compounds with a suitable halogenating agent to form the corresponding acyl halide and subsequentlyreacting the acyl halide with an alkanol, aryl alcohol or aralkanol to obtain the desired ester. The acyl halides of the 6,8-(hydrocarbon substituted mercapto)-S-oxocaprylates may be conveniently prepared by known methods such as by reacting the acid with halogenating agents such as thionyl chloride, phthalyl chloride, phosphorous trichloride, thionyl bromide and phosphorus tribromide in the presence of conventional solvents or an excess of the halogenating agent if it is a liquid. This halogenation may be achieved at 010 C. or at a more elevated temperature if desired. After the acyl halide has been prepared it is recovered from the reaction mixture and esterified'by reaction with an excess of the appropriate alcohol under acidic conditions. The esters formed in this manner are recovered from the reaction mixture by ordinary procedures.

Examples of some 6,8-(hydrocarbon substituted mercapto)-S-oxocaprylic acid esters which may be prepared from the corresponding acid and intermediate acyl halides such as the bromide and chloride that might be mentioned are methyl-6,8-bis(methylmercapto) -5-oxocaprylate, methyl-6,8-bis(ethylmercapto)-5-oxocapry1ate, ethyl-6,8-bis(ethylmercapto)-5-oxocaprylate, benzyl-6,8- bis(propylmercapto) -5-oxocaprylate, propyl-6,8-bis(butylmercapto)-5-oxocaprylate, phenyl 6,8 bis (phenylmercapto) -5-oxocaprylate, methyl-6,8-bis (benzylmercapto 5 oxocaprylate, methyl-6,8-bis(allylmercapto) -5-oxocaprylate, ethyl-6,8-bis(propargylmercapto)-5-oxocaprylate, methyl 6 ethylmercapto 8 propylmercapto 5- oxocaprylate and the like. 7

Lower alkyl esters are also conveniently produced by reacting a 6,8-(hydrocarbon substituted mercapto)-S-oxocaprylic acid with a diazoalkane, such as diazomethane 0r diazoethane, in a suitable dry inert solvent such as ether, chloroform or benzene. Esters may also be produced by reacting the acid With a suitable alcohol under acidic conditions according to the classical method of producing esters. The resulting esters, like the acids are water insoluble oils which may be purified by distillation under reduced pressure. I

In addition to esters, salts of the 6,8-(hydrocarbon substituted mercapto)-S-oxocaprylic acids may also be produced. For example, the alkali metal and alkaline earth metal salts may be prepared by contacting a suitable organic solvent solution of a 6,8-(hydrocarbon substituted mercapto)-S-oxocaprylic acid with an aqueous solution of an alkali metal or alkaline earth metal hydroxide, bicarbonate or carbonate. The salt is readily recovered from the aqueous phase by evaporation to dryness. In this manner, salts such as the sodium, potassium, calcium and magnesium salts of 6,8-(hydrocarbon substituted mercapto)-S-oxoeaprylic acids, such as those specifically named hereinbefore, may be prepared.

According to a further embodiment of this invention diesters of 6,8-(hydrocarbon substituted mercapto)-4-carboxy-S-oxocaprylic acids may also be produced by reacting an ester of a 4,6-(hydrocarbon substituted mercapto)- 3-oxocaproic acid with an ester of a ,B-halo propionic acid. This reaction may be illustrated as follows:

ll CHzCHzCHC CHCHzCHzC O O R R S R 00R wherein R and R represent the same or diiferent hydrocarbon substituents such as alkyl, alkenyl, alkynyl, aryl and aralkyl groups, and R and R represent the same or difierent alkyl, aryl, and aralkyl groups, and X is halogen.

This reaction is preferably efiected by contacting the together with about an equimolar quantity of an enolizing agent such as sodamide, sodium triphenylmethyl and sodie 7 um and potassium alkoxides such as methoxide or ethoxide. v The reaction is conveniently accomplished at elevated temperatures such as the reflux temperatureof the mixture. Under such conditions the reaction is ordinarily completed in from about 1 to hours, usually two hours being entirely adequate. Following completion of the reaction the mixture is preferably acidified with a mineral acid such as sulfuric acid or hydrochloric acid to decompose any excess base present and to aid the recovery of the desired product. The product may then be recovered according to the ordinary techniques.

This reaction is conveniently accomplished employing esters of ,d-h'alo propio nic acid in which the ester substituent is an alkyl, aryl or aralhyl group such as methyl, ethyl, propyl, phenyl and benzyl radicals and in which the fo -halo substituent is chlorine or bromine.

The application of this alternative procedure may be conveniently used to prepare those diesters of 6,8-(hydro carbon substituted -mercapto)-4-carboxy-5-oxocaprylic acids previously disclosed herein and other similar novel compounds. p

The 6,8 (hydrocarbon substituted mercapto)-5-oxocaprylic acids produced according to-the present invention may be converted to 5-[3-(1,2-dithiacyclopentyl)l pentanoic acid, also called u-lipoic acid, by the utilization of a process which is the joint invention of Holly and Wagner and is disclosed in United States patent application Serial No. 396,333, filed December 4, 1953. This process comprises essentially reacting a 6,8-(hydrocarbon substituted mercapto) 5 oxocaprylic acid with an alkali metal borohydride to produce the corresponding 6,8-(hydrocarbon substituted mercapto)-5- hydroxy caprylic acid which immediately forms the corresponding 5 membered lactone, reducing said lactone with phosphorous-iodine to produce a 6,8-(hydrocarbon substituted mercapto) caprylic acid, and reacting said compound with dealkylating agents such as 50% sulfuric acid or thiouronium bromide and oxidizing the resulting material with Iz-KI to obtain 5-[3-(l,2-dithiacyclopentyl)] pentanoic acid.

In the practice of the present invention, the ot,'y(l1yd10- carbon substituted mercapto) butyryl halides which are used as starting materials may be conveniently prepared according to novel methods disclosed in the United States patent application of Arthur F. Wagner, Serial No. 369,535, filed July 21, 1953.

EXAMPLE 1 Tertiary butyl 4,6-bis(methylmercapto)-2-carbethoxy- 3-0x0capr0ate A suspension of 0.10 mole of magnesium ethylate in 100 ml. of anhydrous ether is stirred While 0.10 mole of ethyl t-butyl-malonate is added dropwise over a period of 5 minutes. The mixture is then refluxed for 15 minutes. To the resulting solution of ethyl t-butyl ethoxy magnesium malonate is added a solution of 0.12 mole of a,' -bis(rnethylmercapto) butyryl chloride in 75 ml. of other over a minute period. The mixture is then refluxed for minutes and cooled. The mixture containing t-butyl-4,6-bis(methylmercapto) 2 carbethoxy 3 oxocaproate is diluted with 60 ml. of water and acidified with ml. of 2 N sulfuric acid. The ether phase is separated and the aqueous phase is extracted with ether. The combined ether extracts are Washed with Water, dried over anhydrous magnesium sulfate, and the ether re moved. Final traces of water are removed by azeotropic distillation with benzene. I

The u,' -bis(methylmercapto) butyryl chloride used in this example may be conveniently prepared according to the following representative procedures using oxalyl chloride and thionyl chloride as the halogenating agents.

About 10.7 g. or sodium a,' -bis(methylmercapto) butyrate is suspended in 3-5 ml. of dry benzene and cooled in an ice bath. To this mixture is added 10.2 g. of

oxalyl chloride over a 10 minute period and the mixture stirred for one hour at icebath temperatures. The mixtu're i's'thenevaporated under reduced pressure at a temperature below 15 C. The residue is treated with two 75 ml. portions of ether, the ether extracts filtered and evaporated under reduced pressure to give a,' -bis(methylniei'capto) butyryl chloride as an oil.

B About 13.29-g. of a,'y-bis(methylmercapto) butyric acid is cooled in an ice bath and 9.7 g. of thionyl chloride added thereto. After-standing at about 0 C. for 1 hour the mixture is evaporated at room temperature under reduced pressure to remove thionyl chloride and volatile side products. The a,'y-bis(methylmercapto) butyryl chloride is obtained as an oil.

EXAMPLE 2 Tertiary butyl-4, 6 -bis(t-butylniercapto) -2-c zrbe th0xy- 3-0xocapr0ate In ml. of anhydrous ether 18.8 g. of ethyl t-butyl malonate is reacted with 11.4g. of magnesium ethylate to produce ethyl t-butyl ethoxyrnagnesium malonate. To this solution is then added 33. 8 g. of a,- -bis(t-butylmercapto) butyryl chloride in 100 ml. of ether and the mixture refluxed for 1 hour. Tertiary butyl-4,6-bis(tbutylmercapto)-2 carbethoxy-3-oxocaproate is obtained and is isolated as in Example 1;

EXAMPLE 3 Tertiary 'butyl-4;6-bis(allylmercapt0)-2-carbethoxy-3- oxocaproate To 200 ml. of anhydrous ethyl ether is added 37.6 g. or ethyl t-but'yl 'malonate and 22.8 g. of magnesium ethylate; The mixture is refluxed for 30 minutes and to the solution of ethyl t-butyl ethoxymagnesium malonate is added 55 g. of aq-bistallylmercapto) butyryl chloride in 100 ml. of ether. The mixture is refluxed for 30 minutes, cooled and t-butyl-4,6-bis(allylmercapto)-2-carb'ethoxy-3-oxocaproate' isolated as described in Example 1.

EXAMPLE 4 Tertiary butyl-4,6-bis(benzylmercapto -2-carbometh- 'oxy-3 0xocapr0ate Following the procedure of Example 1, 26.1 g. of methyl t -butyl malonate is reacted with 17.1 g. of magnesium ethylate in 100 ml. of ether to form methyl t-butyl ethoxymagnesium malonate. The mixture in turn is combined With 59.6 g. of org-bis(benzylmercapto)butyryl chloride in 100 ml. of ether and reacted according to this procedure to produce t-btuyl-4,6-bis(benzylmercapto)-2- carbo'methoxy-3-oxocaproate.

EXAMPLE 5 Ethyl-4,6-bis(methylmercapto)e3-0x0capraate After substantially all the water is removed from the t-butyl-4,6 bis'(methylmercapto) 2 carbethoxy 3 oxo capro'a'te prepared as in Example 1 the benzene solution is allowed to cool and 1 gm. of anhydrous p-toluene sulfoni'c acid is added. The acidic solution is refluxed for 1 hours 'afterthe initial vigorous evolution of isobutylone and carbon dioxide has ceased. The benzene solution is then washed with saturated aqueous sodium bicarbonate and 'with water. The benzene solution is dried over magnesium sulfate and the benzene removed by evaporation under reduced pressure. The oily residue is distilled to 'obtaini ethyl-d,'y-bis(methylmercapto)-3-oxocapr'oate; B. P. -114 C./.05 mm.; n =1.5O88; A maxf=2940 A. (13%:480) and 2460 A. (E%=1220); mol wt. found 255:3(calc. 250).

6 Ethyl-'4;6-'bis(t butylmercapto) -3-0x0caproate To 30 g. of 't-butyl- 4-, 6-'bis(t-butylmercapto) -2-carbethoxy-3-oxocaproate in 100 m1. of anhydrous benzene 9 is added 1.5 g. of p-toluenesulfonic acid and the mixture refluxed for 2 hours. Ethyl-4,6-bis(t-butylmercapto)-3- oxocaproate is formed and recovered as in Example 5.

- EXAMPLE 7 Ethyl-4,6-bis(allylmercapto) -3-oxocaproate EXAMPLE 8 Tertiary butyl-4 ,6 -bis( ben'zylmercap to) -3-oxocaproate About 40 g. of t-butyl-4,6-bis(benzylmercapto)-2- carbomethoxy-S -oxocaproate is dissolved in 150 ml. of anhydrous benzene and 2 g. of p-toluenesulfonic acid is added. Reaction is effected as in Example to produce t-butyl-4,6-bis(benzylmercapto)-3-oxocaproate which is recovered as therein disclosed.

EXAMPLE 9 M ethyl-6,8-bis( methylmercapto -4-carbeth0xy-5-0x0- caprylate A solution of 20 g. of ethyl-4,6-bis(methylmercapto)- 3-oxocaproate and 5 g. of 30% methanolic trimethyl benzyl ammonium hydroxide (Triton B) is stirred while 7 g. of methyl acrylate is added over a 15 minute period. During this addition the temperature rises to about 35 C. and soon increases to 50 C. The reaction mixture is held at about 50 C. for 48 hours after which it is cooled and extracted with chloroform. The chloroform solution is washed with 50 ml. of 1N hydrochloric acid, Water and finally dried over anhydrous magnesium sulfate. The other is removed and the residual oil is distilled to obtain methyl-6,8-bis(methylmercapto)-4-car bethoxy-S-oxocaprylate.

EXAMPLE 10 Methyl-6,8-bis(t-butylmercapto) -4-carbethoxy-5-oxocaprylate A methanol solution of 25.1 g. of ethyl-4,6-bis(tbutylmercapto)-3-oxocaproate is prepared and to it is added 6.4 g. of methyl acrylate and 4 g. of 30% methanolic trimethyl benzyl ammonium hydroxide. The mixture is maintained at about 50 C. for 20 hours and the methyl-6,8-bis(t-butylmercapto) -4 carbethoxy 5 oxocaprylate recovered by the procedure of Example 9.

EXAMPLE 11 Methyl-6,8-bis(allylmercapto) -4-carbethoxy-5-oxocaprylate To a solution of 36 g. of ethyl-4,6-bis(allylmercapto)- 3-oxocaproate in methanol is added 10.5 g. of methyl acrylate and 7 g. of 30% methanolic trimethyl benzyl ammonium hydroxide. The mixture is kept at about 50 C. for one day and then extracted with ether. The product is worked up in the usual manner to obtain methyl-6,8-bis(allylmercapto) 4 carbethoxy 5 oxocaprylate.

EXAMPLE 12 Methyl6,8-bis(benzylmercapto)-4-carbo-t-but0xy-5-0xocaprylate By reacting 19.4 g. of t-butyl-4,6-bis(benzylmercapto)- 3-oxocaproate with 4.3 g. of methyl acrylate in ethanol in the presence of 3 g. of 30% methanolic trimethyl benzyl ammonium hydroxide according to Example 9 there is produced methyl-6,8-bis(benzylmercapto)-4- carbo-t-butoxy-5-oxocap1ylate which may be isolated according to conventional procedures.

' 1 0 EXAMPLE 13 6,8-bis(methylmercapt0) -5-oxocapryl ic acid A solution of 6.8 g. of methyl-6,8-bis(methylmercapto)- 4-carbethoxy-5-oxocaprylate' in 100 ml. of glacial acetic acid and'24 ml. of concentrated hydrochloric acid is prepared and allowed to stand at room temperature for 48 hours. The mixture is concentrated under reduced pressure, the residue extracted with aqueous sodium bicarbonate, the extract acidified and extracted with chloro form. The chloroform is removed and the residue is dissolved in 50 ml. of cold 1 N sodium hydroxide and allowed to stand at about 6 C. overnight. The mixture is acidified to pH 3 with concentrated hydrochloric acid and extracted with chloroform. The chloroform is removed and the product heated at -85 C. for 1 /2 hours to eliminate carbon dioxide. The resulting 6,8- bis(methylmercapto)-5-oxocaprylic acid has ultra violet absorption maxima at 2390 A. (E%=27.7) and 2970 A. (E%=15.5).

EXAMPLE 14 6,8-bis( t-butylmercapto) -5-oxocaprylic acid Methyl-6,8-bis(t-butylmercapto)-4-carbethoxy S-oxocaprylate is reacted according to the method of Example 13 to form 6,8-bis(t-butylmercapto)-5-oxocapry1ic acid.

EXAMPLE 1s 6,8-bis( allylmercapto -5-oxocaprylic acid 10 g. of methyl-6,8-bis(allylmercapto)-4-carbethoxy-5- oxocaprylate is added to an aqueous hydrochloric acid solution and the mixture is allowed to stand for one day. The aqueous mixture is extracted with chloroform and by evaporating the chloroform solution 6,8-bis(allylmercapto)-5-oxocaprylic acid is isolated.

EXAMPLE 16 6,8-bis( benzylmercapto) -5-0x0caprylic acid Methyl-6,8-bis(methylmercapto) -4-carbethoxy-5 oxocaprylate using methyl-fl-chloropropionate To a solution of 4.5 g. of sodium methoxide in ml. of methanol is added 20 g. of ethyl-4,6-bis(methylmercapto)-3-oxocaproate. The resulting solution is refluxed and to it is added 9.8 g. of methyl-fl-chloropropionate over a 10 minute period. Refluxing is continued for 2 hours with stirring. The mixture is cooled and acidified to pH 3-4 with a cold solution of hydrochloric acid. The acidified mixture is diluted with ice water and extracted with chloroform. The chloroform extracts are washed with water and dried over anhydrous magnesium sulfate. The chloroform is evaporated and the residual oil distilled to give substantially pure methyl 6,8-bis- (methylmercapto)-4-carbethoxy 5 oxocaprylate; B. P.

160 C./50 mm., n =1.5028.

What is claimed is: 1. A compound selected from the group consisting of compounds having the formula 5. 6,8-bis(aralkylmercapto)--oxocaprylic acid.

6. An alkyl 6,8-bis(a1kyl mercapt0)-5-oxocaprylate.

7. Alkali metal salts or 6,'8-"t 'is(alkylmercapto)-5-oxocaprylic acid.

8. A -6,-8-bis(alkylmercapto-)-5-oxocapryly1 halide.

9. -6,8-'bis(methylmercapto)-5-oxocaprylic acid.

10. '6,8-bis(t-butylmercapto)-5-oxocaprylic acid.

11. 6,8-bis(allylmercapto)-5-oxocaprylic acid.

12. 6,8-bis(benzylmercapto)-5-oxocapry1ic acid.

13. The process for 'the production of compounds having the formula .11 n omomoraowrmwoorr SR SR which "comprises treating-a compound having the formula with a mineral acid selected from the class consisting of hydrochloric acid and sulfuric acid, wherein R and R represent hydrocarbon groups,- and R represents a hydroca'rbon group containing tip to and including eight carbon atoms. f

14. The process of claim 13 wherein the reaction is carried out in the presence of lower carboxylic acid.

15. The process of claim 14 wherein the lower carboxylic acid is glacial acetic aci 16. A process which comprises reacting methyl-6,8-

' bis(methyl-mercapto) 4 carbethoxy 5 oxocaprylate with a mineral acid selected from the class consisting of hydrochloric acid and sulfuric acid to .produce 6=8-bis- (methylmercapto)-5=oxocaprylic acid.

17-. A process which comprises reacting methyl-6,8- bis(t-butyl-mercapto) -4 carbethoxy 5 oxocaprylate with a mineral acid selected fror'nthe class consisting of hydrochloric acid and sulfuric acid to produce '6,8-bis(tbutylmercapto)-5-0xocaprylic acid.

18. A process which comprises reacting methyl-6,8- -bis(ally1r"r iercapto) 4 carbethoxy 5 cxocaprylate with a mineral "acid selected -from the class consisting of hydrochloric acid and sulfuric acid to produce "6,8-bis (allyl'iner'capto) -5-oxocap'r'ylic acid.

19. A process which comprises reacting methyl-6,8- bis(benzylmercapto) '4 carbomethoxy-5-oxocaprylate with a mineral acid selected from the class consisting of hydrochloric acid and sulfuric acid to vproduce 6,8-bis- (benzylr'nercapto) -5-oxocaprylic acid.

References Cited in the file of this patent UNITED S IATES PATENTS 2,408,094 Pavlic Sept. 24, 1946 2,422,246 Lazier et al. June 1-7, 1947 2,568,648 Mc Cool Sept. 18, 1951 FOREIGN PATENTS 867,394 Germany Jan. 18, 1953 990,676 France Sept. 25, 1951 OTHER REFERENCES Hudson and Hauser, Journal of the "American Chemical Society, volume 63, November 1941, pages 3163-4. 

1. A COMPOUND SELECTED FROM THE GROUP CONSISTING OF COMPOUNDS HAVING THE FORMULA 